Preheat the oven to 180°C (350°F). Grease a large Le Creuset Heritage Rectangular Roasting Dish generously with butter or non-stick cooking spray.
Place the chopped figs in a bowl with the bicarbonate of soda. Pour over 400ml of boiling water and stir well. Set aside to soak for 10 minutes.
Using an electric mixer, beat the butter and sugar together until light and fluffy, about 3 to 4 minutes.
Add the vanilla extract. Add the eggs one at a time, beating well after each addition. The mixture may look slightly curdled at this point — that's fine.
Add the flour and mix briefly until just combined. Add the chopped walnuts and the figs along with all of their soaking liquid. Mix until everything is incorporated.
Pour the batter into the prepared dish. Bake for 30 to 35 minutes until springy to the touch and a skewer inserted into the centre comes out clean.
While the pudding bakes, make the toffee sauce. Combine the sugar, butter, cream and vanilla in a small saucepan over low heat, stirring until the sugar dissolves and the sauce is smooth. Add the molasses if using.
As soon as the pudding comes out of the oven, pour half the toffee sauce over the top and leave it to absorb. Return the remaining sauce to low heat and stir for a further 10 minutes until thickened and glossy. Scatter the toasted walnuts over the pudding.
Serve warm with the remaining toffee sauce in a warm jug alongside. Vanilla bean ice cream or whipped cream on the side.